ci: make bootstrap just another kas overlay

Simplify the gitlab-ci by having the bootstrap build as just another Kas
overlay file, so there's no duplication of build script.

Change-Id: I7341750d2ae7f3c146bfe323f61fa98c0f3121c0
Signed-off-by: Ross Burton <ross.burton@arm.com>

kas/bootstrap.yml

@@ -0,0 +1,9 @@
+header:
+  version: 9
+
+target:
+  - binutils-cross-aarch64
+  - gcc-cross-aarch64
+  - python3-native
+  - opkg-native
+  - rpm-native